Several important processes in chemical or food production, waste treatment and recovery or purification operations involve the combination of two or more fluids so that separation, mixing and/or a reaction occurs. Such processes include gas-liquid processes, such as adsorption, absorption, desorption and two-phase reactions, and liquid-liquid processes, such as extraction and reactions. While acceptable equipment exists for performing such processes, deficiencies exist which detract from the efficiency of such devices.
The process of liquid-liquid extraction is well-known in the art, as are extractor devices for performing extraction. In liquid-liquid extraction, one or more components are removed from a liquid solution or mixture feed by intimate contact with a second liquid. The second liquid is immiscible with the liquid mixture, but functions as a solvent for removing the component(s) from the mixture as the second liquid exhibits a preferential affinity or selectivity towards one or more of the components in the mixture feed. Liquid-liquid extraction is therefore a mass transfer operation.
A conventional device widely used for performing liquid-liquid extractions is a sieve tray column, also sometimes referred to as a perforated plate column. Examples of such devices are presented in U.S. Pat. No. 3,988,116 to Robbins and U.S. Pat. No. 4,424,131 to Baird.
The liquid-liquid extraction process that occurs in a conventional sieve tray column 10 is illustrated in FIGS. 1-3. FIG. 1 shows the general flow. Positioned at the top end of the column are a Liquid 2 inlet and a Liquid 1 outlet. Positioned at the bottom end of the column are a Liquid 1 inlet and a Liquid 2 outlet. As illustrated in FIG. 2, the sieve tray column, indicated in general at 10, features a number of perforated plates, two of which are illustrated at 12 and 14. In the example illustrated in FIGS. 1-3, a solvent, Liquid 2 in FIG. 1, flows through the inlet at the top of the column, down through the column, and out the outlet at the bottom of the column. A solution, Liquid 1 in FIG. 1, having the component to be removed flows into the column through the inlet at the bottom of the column, up through the column, and out the outlet at the top of the column. In this example, the density of Liquid 2 is greater than the density of Liquid 1, but another case could occur if the solution liquid were denser. Then that liquid would enter at the top and the bubbles would flow downward.
As illustrated in FIG. 2, a liquid bubble 16 of solution is formed as solution (Liquid 1 of FIG. 1) rises through the opening or hole of the lower distributing tray 14. The mass transfer of a component A, which is the target of the extraction, involves several steps. Molecules of the component A move into the solvent (Liquid 2 in FIG. 1) by mass transfer from the surface of the bubble due to the solubility difference in the two liquids. These molecules continue migrating into the solvent (Liquid 2) by diffusion. The decrease in concentration of component A at the surface of the bubble leads to a concentration gradient within the bubble. While the bubble is rising due to buoyancy, as illustrated by arrow 18, the key component A within the bubble of solution (Liquid 1) is diffused from inside of the bubble (as illustrated by arrow 20 in FIG. 3) to interface with solvent (Liquid 2) outside of the bubble through the gradient of concentration of A. The transfer is limited by equilibrium considerations.
When bubbles reach the underside of the upper tray 12, their movement is impeded and they will rejoin to form a new layer of solution (Liquid 1) with a uniform concentration of component A. The layer is called a rejoining layer and is illustrated at 22 in FIG. 2. New small liquid bubbles will be formed again through the perforations of upper distribution tray 12, and the above process repeats.
The mass transfer of component A to the solvent outside of the bubble 16 occurs by diffusion due to the gradient of concentration of component A at a boundary of the bubble. With reference to FIG. 3, the gradient in the boundary area of bubble 16 may be calculated by the equation (CAb−CA)/b, where:                CAb=concentration of component A at the inner edge of the boundary        CA=concentration of component A at the outer edge of the boundary (which is the concentration of component A in the solvent)        b=the thickness of the boundary        
If the transfer through the boundary area is faster, then CAb and CA can reach equilibrium sooner. The transfer from inside of the bubble to the inner edge of the boundary, that is, the transition from concentration CA0 to concentration CAb (where CA0 is the concentration of A at the center of the bubble), is typically the rate control (limiting) step because it is slower than the transfer through the boundary area. The mass transfer from CA0 to CAb highly depends on diffusion which has a high mass transfer resistance. As a result, CAb increases slowly giving a low mass transfer efficiency due to the lower gradient in the boundary area of the bubbles (per the above equation).
Turning to an example involving a gas-liquid process, absorption or desorption is typically performed using a packed bed tower filled with high surface area or high efficiency packing materials. The efficiency from current packed bed devices, as described above with regard to the liquid-liquid extraction column, is also restricted by the limitation of mass transfer within the bubbles or droplets formed in the tower.
The formation of smaller bubbles or droplets in either the liquid-liquid or gas-liquid devices described above would help to provide more surface area per volume of material and a reduction of the diffusion distance in the bubbles or droplets. A need therefore exists for a fluid processing device that can provide a large surface area via the formation of smaller bubbles or droplets for mass transfer, and also provide convection mass transfer via mixing within bubbles or droplets through frequently separating and rejoining the bubbles or droplets.